1. Field of the Invention
The present invention relates to a coordinate input apparatus and, more particularly, to a coordinate input apparatus for detecting an input vibration from a vibration pen by a vibration sensor provided on a vibration transmitting plate to measure a vibration transmission time to the vibration pen to detect the input coordinates of the vibration pen on the vibration transmitting plate on the basis of the measurement result.
2. Related Background Art
Conventionally, as an apparatus for inputting handwritten characters or graphics to a processing apparatus, such as a computer, coordinate input apparatuses using various input pens and tablets are known. In an apparatus of this type, image information consisting of input characters or graphics is displayed on a display unit, such as a CRT display, or output to a recording unit, such as a printer.
Various systems described below are known as a coordinate detecting system for use in a tablet-type coordinate input apparatus as one of coordinate input apparatuses of the above type.
(1) A system which is constituted by resistance films and a sheet member arranged to oppose the resistance films to detect the coordinate values of a depressed point in accordance with a change in resistance at that point.
(2) A system for detecting a coordinate position on the basis of electromagnetic or electrostatic induction between conductive sheets or the like arranged to oppose each other.
(3) A system for detecting the coordinate position of an input pen on the basis of an ultrasonic vibration transmitted from the input pen to a tablet.
Conventional coordinate input apparatuses using the above systems, however, have the following drawbacks. That is, in the system (1) using resistance films, since the uniformities of the resistors have a direct influence on a graphic input accuracy, a resistor with a high uniformity is required. Hence, an apparatus required to have a very high accuracy becomes very expensive.
In addition, since two resistance films for X and Y coordinates are necessary, the transparency of a coordinate input surface is reduced. Therefore, when the apparatus is used by overlapping it on, e.g., an original, the readability of the original is degraded.
In the system (2) using electromagnetic induction, electric wires are arranged in a matrix manner, so the coordinate input surface of the apparatus is not transparent. Therefore, the apparatus is unsuitable for an application in which it is overlapped on an original or a display unit. Also, since the coordinate detection accuracy of an apparatus adopting this system is affected directly by the positions of electric wires arranged in a matrix manner, i.e., by the fabrication accuracy, an input apparatus with a high accuracy is very expensive.
Unlike these two systems, the system (3) using an ultrasonic wave detects the delay time of a wave propagating on a tablet as an input surface, thus calculates position coordinates. Therefore, since components, such as matrix-like electric wires, are not formed on the tablet, an inexpensive apparatus can be provided.
In addition, when transparent plate glass is used as the tablet, it is possible to fabricate a coordinate input apparatus with a higher transparency than those of other systems.
In conventional ultrasonic coordinate input apparatuses, however, a delay time until arrival during which a vibration produced by an input pen propagates on the tablet from the input point to a sensor is detected to calculate position coordinates. Hence, it is necessary to know a reference time, i.e., a time at which the vibration is produced by the input pen.
For this reason, time information indicating a time at which a vibration is generated by the input pen must be supplied constantly to a circuit for detecting the delay time until arrival, so it is necessary to connect the input pen for generating a vibration to a main body through a cable or the like. This results in a poor operability when this coordinate input apparatus is used to input characters or graphics.
To transmit the time information signal on, e.g., a radio wave, on the other hand, a transmitter or the like must be incorporated in the pen. This makes it difficult to realize an input pen with a shape, a weight, and operability similar to those of ordinary pens or pencils. In addition, when switches required for intended applications are additionally provided in the input pen, the structure of the pen is further complicated and enlarged, resulting in a poorer operability.
The ultrasonic system also has the following problem. That is, a vibration input from the input pen to the tablet propagates in the tablet, though it is damped, when it reaches the end face of the tablet, and it is reflected by the end face. This reflected wave cannot be perfectly eliminated even when a vibration insulator is attached to the end face to suppress the vibration. Therefore, a direct wave (a wave propagating through the shortest route from the input pen to a sensor for detecting a vibration) required to measure the delay time until arrival of a wave and the reflected wave are superimposed, and the waveshape of an output detection signal from the sensor is distorted by this superimposed portion. The distortion causes an error in measurement of the wave propagation time, and this largely decreases the coordinate detection accuracy. To solve this problem, an apparatus must be arranged such that no reflected wave is superimposed on the delay time detection point in a detection signal waveshape formed by the direct wave. For this reason, sensors are located at positions free from the influence of a reflected wave.
This method, however, solves the above problem by increasing the difference in route between the direct wave and the reflected wave to prolong a time in which the reflected wave returns. This consequently leads to another problem that the size of the coordinate input surface is increased relative to the size of the overall apparatus, i.e., the size of an effective area capable of coordinate input.
In addition, as the sonic velocity of a wave propagating in the tablet increases, the resolution of coordinate calculation decreases. This means that as the sonic velocity of a wave to be handled decreases, the resolution of distance measurement improves, provided that the resolution of a counter for measuring a time is the same. Hence, when a distance is to be calculated from the delay time of a wave with a high velocity, it is necessary to use a counter with a time resolution (or it is necessary to increase the frequency of a counter) high enough to maintain the resolution of distance calculation. The results are high cost and high power consumption.